Internal combustion engines exhaust a complex mixture of chemical species. These chemical species may include gaseous and solid materials, including particulate matter, nitrogen oxides (“NOx”), and sulfur compounds.
Due to heightened environmental concerns, exhaust emission standards have become increasingly stringent over the years. The amount of chemical species emitted from an engine may be regulated depending on the type, size, and/or class of engine. One method that has been implemented by engine manufacturers to comply with the regulation of particulate matter exhausted to the environment has been to remove these species from the exhaust flow of an engine with filters. These filters may include filter media to capture and oxidize the particulate matter pollutants contained in the exhaust.
There are several types of filter media that are either available or in development. Wall-flow ceramic cordierite, woven fiber cartridges, and disposable temperature resistant paper are three common types of filter media that are currently in use commercially. Also, knitted silica fiber coils, ceramic foam, wire mesh, and sintered metal substrates are all filter media that have been tested. Most of the filters employing such filter media operate by a similar process of forcing engine exhaust through the filter media which blocks the particulate matter in the exhaust on the inflow side of the media. Using these filters for extended periods of time may cause the particulate matter to buildup in the filter media, impeding the flow of gas through it, resulting in increased pressure drop within the filter and reduced engine efficiency.
Using disposable filter media and filter regeneration are two ways to remove the particulate build up within the filter media. Regeneration is the process of increasing the temperature of the exhaust system until the organic components of the particulate matter such as the soot and the soluble organic fraction (SOF) that accumulated in the filter burn off. If the engine exhaust does not reach the temperature required for regeneration within the filter, an additional component is necessary to raise the temperature within the filter. In some systems this component is an outside heat source that heats the filter media or the engine exhaust before it reaches the filter. A catalyst is sometimes used to lower the regeneration temperature necessary to oxidize the soot and the SOF. For example, some filters include a filter media coated with a noble or base catalyst material, while others include a catalyst upstream of the filter, or include fuel borne catalysts. Both precious and base metals have been used as catalysts in filters. Since pressure drop across a filter results in decreased engine efficiency, the catalyst coating should avoid significant pressure drop across the filter.
U.S. Patent Publication No. US 2006/0057046 A1 (the '046 publication) to Punke et al., describes a catalyzed soot filter with its internal walls coated with different catalyst compositions. The catalyzed soot filter of the '046 publication consists of a conventional ceramic wall flow filter media of a honeycomb structure, with a washcoat containing a platinum group metal or a rare earth metal oxide catalyst coated on its internal walls. The '046 publication discloses coating of the upstream zone and the downstream zone with washcoats with different densities of catalyst loading to account for the non uniformity of the amount of particulate matter accumulating along the length of the filter. The washcoat catalyst loadings in the '046 publication are tailored to account for the fact that a higher proportion of the particulate matter is deposited on the downstream side of the filter. Since a higher proportion of the particulate matter is deposited on the downstream side, this region of the filter will experience the high temperatures during regeneration more than the upstream side. Therefore, the durability of the catalyst coating on the downstream side will limit the useful lifetime of the filter. Thus, the '046 publication seeks to increase the useful lifetime of the filter media by increasing the durability of the metal catalyst which will be exposed to high temperatures during regeneration.
High regeneration temperatures deteriorate the filter media in a filter, limiting the useful life of the filter media. Catalyst coatings on the filter media have the potential of increasing the pressure drop in the filter, with a resulting decrease in engine efficiency, by increasing the resistance to exhaust flow through the filter. The present disclosure is directed to solving one or more of the problems set forth above.